ARILLUS, an improper term invented by Linnaeus, and defined to be the proper exterior coat or covering of the feed which falls of spontaneously.
All feeds are not furnished with an arillus; in many, a dry covering, or scarf-skin, supplies its place. In Jessamy; hound's tongue, cynoglossum; cucumber; fraxinella, dicranum; flax-tree, celastrus; spindle-tree, euonymus; African spirea, diosma; and the coffee-tree, coffee; it is very conspicuous.
In the genus hound's tongue, four of these arilli, or proper coats, each enfolding a single feed, are affixed to the stylus; and in this circumstance, says Linnaeus, does the essence of the genus consist. In fraxinella, the arillus is common to two feeds. The flax-tree has its seeds only half involved with this cover.
The arillus is either baccatus, succulent, and of the nature of a berry; as in the spindle-tree, euonymus. Cartilagineus, cartilaginous, or grizzly; as in the African spirea, diosma. Coloratus, coloured; as in the flax-tree. Elasticus, endowed with elasticity, for dispersing the seeds; as is remarkable in the African spirea, diosma, and fraxinella. Staber, rough and knotty; as in hound's tongue.
Although covered with an arillus or other dry coat, seeds are said to be naked (semina nuda) when they are not inclosed in any species of pericarpium or fruit-vessel; as in the grasses, and the labiati or lipped flowers of Tournefort, which correspond to the didynamia gymnospermia of Linnaeus. Seeds are said to be covered (semina tecta) when they are contained in a fruit-vessel, whether capsule, pod, or pulpy pericarpium, of the apple, berry, or cherry kind: (See SEMEN). This exterior coat of the seed is, by some former writers, styled calyptra. See CALYPTRA.
The different skins or coverings of the feed, are adapted, say naturalists, for receiving the nutritive juices, and transmitting them within.